The internal capsule is a structure of the brain made up of fibers with myelin, Through which pass nerve projections from the cortex to the spinal cord and subcortical structures, and vice versa.
In this article, we explain in more detail what the internal capsule is, what is its composition and anatomical structure, what are the functions of the most important nerve pathways and what are the consequences of injuries on this region of the brain.
What is the internal capsule of the brain?
The internal capsule is a region of the brain formed by a small band of myelinated fibers, also called white matter, Which separates the lenticular nucleus from the caudate nucleus and thalamus. This substance is made up of projections going from the cerebral cortex to different subcortical structures.
this structure it consists of an anterior arm and a posterior arm, as well as ascending and descending projections. Precisely, when these circulate between the lenticular nucleus and the area of the thalamus and the caudate nucleus, the material compacts giving rise to the internal capsule.
This area of the brain contains vitally important pathways, such as the corticobulbar and corticospinal pathways. In fact, specific damage to the inner capsule (which can occur, for example, as a result of strokes such as heart attacks or strokes) can lead to devastating clinical deficiencies for the affected person.
The internal capsule it does not form one of the basal ganglia, but a bundle of fibers running through them. The shape of the internal capsule varies depending on how we analyze the brain. If we make a frontal cut, this brain structure takes the form of an outwardly open angle with a lower horizontal band, under the lenticular nucleus.
If we make a horizontal cut, it has a V-shaped appearance, where the knee (its end) points medially, and we can divide it into three basic components:
The anterior arm
The anterior arm or the lenticular-caudate part of the internal capsule separates the lenticular nucleus from the caudate nucleus.
It contains fibers which run from the thalamus to the crust (thalamocortical) and, conversely, from the crust to the thalamus (corticothalamic), which unites the lateral thalamic nucleus to the crust of the frontal lobe; the frontopontine relations of the frontal lobe with the pontine nucleus; already fibers passing transversely from the caudate nucleus to the putamen.
The posterior arm
The posterior arm or lenticulo-thalamic part of the internal capsule, located between the thalamus and the lenticular nucleus, Contains the essential ascending and descending pathways.
The corticobulbar and corticospinal tracts travel through the anterior half of the posterior arm, with the fibers in the face and arm, opposite the fibers in the leg. Corticorrubral fibers, which originate from the cortex of the frontal lobe in the red nucleus, accompany the corticospinal tract.
The posterior third of the posterior arm is made up of third-order sensory fibers ranging from the posterolateral nucleus of the thalamus to the post-central gyrus (where the somatosensory cortex is located, an area of sensory reception, such as touch).
As in the case of the corticospinal and corticobulbar fibers, there is a somatotopic organization of the sensory fibers of the posterior arm, the face and the arm rising in front of the fibers of the leg.
Composition of its nerve fibers
The inner capsule is made up of white matter, i.e. nerve fibers covered with myelin, a substance that surrounds and protects axons, allowing a greater speed of transmission of nerve impulses.
Depending on the length of the path traveled by each of the fiber bundlesWe can divide them into short and long fibers.
This type of fiber binds to the thalamus with various areas of the cerebral cortex (thalamocortical fibers). They emerge along the entire ovoid mass of gray matter that makes up the thalamus and, seen at the macroscopic level, it seems that they radiate in all directions (hence the name thalamic radiations).
The anterior thalamic peduncle is made up of fibers that emerge from the anterior part of the thalamus and go to the frontal lobe, occupying the anterior arm of the inner capsule. The upper peduncle, on the other hand, emerges from the upper and posterior part of the thalamus to go to the parietal lobe, passing through the posterior arm of the internal capsule.
On the other hand, the fibers that connect the posterior area of the thalamus to the occipital lobe they form the posterior peduncle, which also runs along the posterior arm of the internal capsule. And finally, the fibers emerging from the anterior part of the thalamus and projecting into the temporal lobe occupy the sublenticular region of the inner capsule and form the lower peduncle.
This other type of fiber they are responsible for connecting the cerebral cortex with nerve areas further away from the brain, Passing through the internal capsule and occupying the anterior arm, knee and posterior arm.
These fibers constitute: the frontopontin fascicle; the geniculate fascicle, responsible for voluntary movements of the head; the pyramidal route, responsible for voluntary movements of the trunk and limbs; the sensory path; the parieto-pontine and occipito-pontic fascicles; and the retrolenticular part or Wernicke corridor, an area of great importance both motor and sensitive, and in which the optical and acoustic radiation intersect.
The internal capsule is a region of the brain in which two very important bundles of nerve fibers converge: the corticospinal tract and the corticobulbar tract. Then we will see what functions each of them performs.
The corticospinal tract
The tract or corticospinal tract is a collection of nerve fibers that are part of the internal capsule and they are responsible for controlling the voluntary movement of the body. It is essential to manage the execution of fine movements (with the fingers of the hand, for example). Its function is to ensure that these movements have the proper skill and precision.
On the other hand, it is also involved in the regulation of sensitive reliefs and when selecting the sensory modality that ultimately reaches the cerebral cortex. The corticospinal tract stimulates the neurons responsible for flexion and inhibits those responsible for extension.
In the corticospinal tract, there is a somatotopic representation of different parts of the body in the primary motor cortex, with each area of the lower limb being located in the medial cortex and the area of the cephalic limb located in the cortex. lateral, to the convexity of the cerebral hemisphere (motor homunculus).
The motor area of the arm and hand is the one that occupies the largest space (Occupying the precentral gyrus, located between the lower limb area and the face).
The corticobulbar tract
The corticobulbar tract or tract is a bundle of nerve fibers that is responsible for directing the muscles of the head and neck. This nervous system is responsible for the fact that we can control, for example, facial expression, or that we can chew or swallow.
The tract develops in the lateral part of the primary motor cortex, and its fibers eventually converge in the inner capsule of the brainstem. From there they go to the motor nuclei of the cranial nerves and these connect to the lower motor neurons to innervate the muscles of the face and neck.
Consequences of damage in this region of the brain
Injury in an area of the brain such as the inner capsule can compromise motor and sensory function selectively. For example, lacunar infarctions, strokes less than 15 mm in diameter and caused by occlusion of perforating arteries in the brain, can selectively compromise the part. anterior of the posterior arm of the internal capsule, producing pure motor hemiparesis.
Heart attacks or strokes can cause another set of symptoms and syndromes, Depending on the affected area. Pure sensory syndrome is another condition that occurs during the production of heart attacks located in the capsule. inner and / or posterior nucleus of the thalamus. The affected person, in this case, suffers from facio-brachio-crural hemi-hypoesthesia (decreased sensitivity in almost half of the body).
Another consequence associated with damage to the cerebral blood vessels, which causes an infarction or thrombosis in the posterior arm of the inner capsule, is contralateral hemiplegia (on the opposite side of the body), due to an interruption of the corticosteroid fibers. spinal muscles that run from the crust to the spinal cord and muscles that perform motor functions.
Finally, there are two other disorders associated with specific lesions of the internal capsule and associated structures. On the one hand, clumsy hand dysarthria syndrome, For internal capsule and running knee pain with symptoms such as facial weakness, manual clumsiness, dysphagia and dysarthria; and on the other hand, paresis with hemiataxis, when the cortico-ponto-cerebellar pathway and the posterior arm of the internal capsule are affected, and which is accompanied by symptoms such as paralysis and incoordination of the leg or the arm.
- Pendlebury, ST, Blamire, AM, Lee, MA, Styles, P. and Matthews, PM (1999). Axonal injury to the inner capsule correlates with motor impairment after stroke. Stroke, 30 (5), 956-962.
- Snell, RS (2007). Clinical neuroanatomy. Pan American Medical Ed.